In military ordnance arts, destructive devices commonly referred to simply as "warheads," have been developed to accomplish a wide variety of military mission requirements.
A shaped charge warhead refers to a generally axially symmetric combination of components including, among others, a liner designed to collapse upon explosive detonation and form a directed-energy penetrator, an explosive material or charge, a firing or explosive initiation mechanism intended to detonate the explosive charge and thereby forcibly propel the penetrator toward a target, and a warhead housing in which the liner and explosive charge are confined before firing. A delivery vehicle commonly carries the warhead to the target.
Shaped charge warheads are based upon the hollow charge principle, which has been known since before 1800. In fact, the French charge creuse or the German Hohlladung, which mean literally "hollow charge," are more descriptive than the English. A demonstration of an ability of hollow charges to direct explosive energy was provided independently by von Foerster and Munro in the 1880s. The potential for military applications of this directed explosive energy led to extensive research during early years of the twentieth century. It was discovered that when an axially symmetric explosive charge is hollowed out at one end to form a "charge cavity", and the charge cavity placed against a metal target block, and the charge is initiated at its opposite end, a deep hole is produced in the target. In 1938 Thomanek in Germany discovered that if the cavity is lined with metal, penetration of the charge is magnified. Charges with lined cavities, commonly referred to as simply "shaped charges", were used widely during World War II. The United States "Bazooka" and the German Panzerfaust were two anti-armor weapons employing shaped charges.
The hollow charge principle may be characterized as follows. Explosive detonation products or gases, expanding into the charge cavity, converge on a central charge cavity axis due to axial symmetry. The converging gases divide into two flows, one directed forward and away from the charge, and one being directed back into the charge. Due to geometry of the converging flows and laws of mass and momentum conservation, the forward-directed gases acquire a much greater velocity than the backward-directed gases. This high velocity jet of gases strikes the target, creating a pressure high enough to erode the target material to a considerable depth.
A further refinement of the aforesaid hollow charge principle includes the addition of a metal liner in contact with the explosive charge which forms the charge cavity--i.e., the shaped charge liner is configured to mate with the formed charge cavity or more typically, the charge is formed or cast over the liner. When the charge cavity is lined with a metal liner such as copper, the same basic hollow charge principle still applies. In a lined cavity, energy of a detonated explosive is transferred to the shaped charge liner as a detonation wave sweeps over the shaped charge liner from the rear or aft portion of the shaped charge liner. The shaped charge liner is subjected to such high stresses that it begins to behave more as a fluid than as a solid. In turn, a high velocity jet is created in the same manner as described above, except that the jet consists of metal, herein referred to as a "metal jet," and is able to direct energy at a small area of an intended target. The force of the impinging metal jet is sufficient to erode a target such as armor plate, thereby creating a crater in the target, much as a garden hose deeply erodes soil if it is directed at one point for a sufficient length of time. Because of this garden hose effect similarity, this type of armor penetration is often termed "hydrodynamic."
Modern shaped charges have incorporated many refinements in design and materials, but employ the same principles outlined above. Tip velocities in excess of 9000 meters/second have been obtained with metal jets formed of copper liners, and even higher velocities can be achieved with other metals having higher sound speeds. Copper has been most widely used as a shaped charge liner material due to its ductility and consequent ability to produce very long jets.
Shaped charge liner geometry also plays an important role in determining shaped charge performance. Trumpet or horn shaped charge liners, for example, are able to produce very high metal jet tip velocities and deep penetration. Superior shaped charge explosive warhead performance requires high quality materials and precise fabrication techniques.
Research conducted on shaped charge explosive warheads of the prior art has shown that very low lateral ("drift") velocity of the jet is required to achieve deep target penetration. That is, low drift velocity maximizes the amount of metal jet material reaching the lowest point of the crater made in the target by the metal jet, which advantageously contributes to penetration depth into the target.
Drift velocity is dependent on many factors, foremost among them being explosive charge detonation wave and liner concentricity. Detonation wave concentricity in the explosive charge is primarily dependent upon the method of detonating the explosive charge, and secondarily upon the homogeneity of explosive material. The technique commonly employed in shaped charge explosive warheads is the use of a precision fabricated initiation assembly positioned concentrically on the explosive charge axis. The first element of the initiation assembly is typically a detonator containing a small amount of explosive which, upon receiving an electric current, initiates a "train" of explosive elements leading finally to main charge initiation. Each element of the explosive train is initiated by the energy released (output) from the previous element and provides sufficient energy (input) to initiate the next element in the train.
An explosive initiation train for a shaped charge warhead usually incorporates a combination of elements referred to as a precision initiation coupler (PIC)/booster. In this design, the output from the detonator initiates a small diameter column of explosive, the output from which is precisely positioned on the shaped charge axis and is sufficient to initiate a pellet referred to as the booster, also positioned concentric with the axis. Detonation wave concentricity using a PIC/booster has been difficult to achieve consistently in high volume shaped charge production, primarily because the "stackup" of tolerances normally used in fabricating the various components leads to small variations in the position (i.e. eccentricities) of the PIC/booster relative to the explosive charge and liner.